1. Field of the Invention
Embodiments of the invention generally relate to a method and apparatus for temperature sensing utilizing optical fiber.
2. Background of the Related Art
Transmitting information, such as temperature, through optical fibers utilized in down hole gas and oil (e.g., petroleum) field drilling applications is becoming more widely accepted as gas and oil field producers embrace the advantages of optical fiber systems over conventional metallic conductors. For example, optical fiber sensing systems exhibit increased long-term reliability over conventional conductors, often having a useful service life up to and exceeding four times the service life of conventional sensing systems utilizing metallic conductors, thus allowing efficient petroleum removal to continue long into the life of wells utilizing optical sensing systems, and thereby maximizing the profitability of older wells.
One type of fiber optic temperature sensing system exploits the Brillouin shift in reflected wavelengths (or frequencies) of high powered optical signals traveling in optical fibers. Such sensing systems generally include a signal generator and detection circuit coupled to an optical fiber housed in a cable suitable for down hole oil and gas field service. A high powered signal is launched down the optical fiber by the signal generator. As the signal propagates through the optical fiber, the intensity of the signal produces a temporary property change in the portion of the fiber having the signal pass through. The property change causes a reflection of the signal back through the optical fiber to the detection circuit.
The optical detection circuit compares the wavelengths (or frequencies) of transmitted and reflected signals to determine the temperature at the portion of the optical fiber from which the signal was reflected. As the rate of signal propagation through the optical fiber is known, the delay between signal generation and the return of the reflected signal is indicative of the position of the portion of the optical fiber reflecting the signal.
As the optical fiber is often routed deep inside the well, the optical fiber is subjected to considerable strain due to environmental conditions within the well along with the weight of the cable itself. As the characteristics (i.e., frequency or wavelength) of the reflected signal are influenced by the strain applied to the optical fiber, optical temperature measuring systems must predict the strain contribution to the characteristics of the reflected signal in order to obtain an accurate temperature reading.
However, predicting the contribution of strain to the wavelength or frequency change of the signal is difficult. For example, the large difference in thermal expansion between pipes and other components to which the optical fiber is housed or coupled to may subject the optical fiber to strain if insufficient slack or binding of the optical fiber occurs in the hot well environment. Moreover, as the temperature is not constant throughout the well, the strain induced by temperature may vary along the length of the optical fiber. Additionally, the temperature at various locations within the well usually changes over time, further complicating predictions of strain within the optical fiber and limiting the accuracy of temperature measurements using optical fiber.
Therefore, there is a need for an improved method and apparatus for sensing temperature using optical fiber.